This application relates to and incorporates by reference Japanese patent application No. 2001-139244 filed on May 5, 2001.
The present invention relates to an EL (electroluminescence) element and a display panel fabricated by use of the EL element used for a self-luminescence type segment display or matrix display of an instrument or a display for various terminal apparatuses.
FIG. 5 shows a vertical cross sectional view of a conventional EL element in general use. The EL element is formed by laminating a first electrode 2, a first insulation layer 3, a luminescent layer 4, a second insulation layer 5, and a second electrode 6 laminated successively on an insulative substrate 1 such as a glass substrate. Furthermore, a counter glass substrate 8 is adhered with adhesive 7 on the EL element to form a display panel.
The insulation layers 3 and 5 are formed of silicone dioxide (SiO2), silicone nitride (SiN), silicone oxide nitride (SiON), or tantalum pentoxide (Ta2O5) by means of sputtering or vapor deposition.
A method in which aluminum oxide (Al2O3) layers and titanium oxide (TiO2) layers are laminated alternately by means of atomic layer epitaxy (referred to as ALE herein) to form the insulation layers 3 and 5 as the Al2O3/TiO2 laminate-structured film has been proposed (refer to JP-A No. S58-206095).
In the case of the Al2O3/TiO2 laminate-structured film, the Al2O3 layer serves as an insulation layer and the TiO2 layer serves as a semiconductor layer, and the laminate structure comprising the insulation layer and the semiconductor layer forms a heavily insulative layer.
It is inevitable that the insulation layers 3 and 5 formed by sputtering, vapor deposition, or ALE contain some defects, or vacancies. As the result, it is difficult to provide a sufficient withstand voltage on all the areas of a display panel (EL display panel). Each area forms a plurality of EL elements for serving as a pixel.
EL display panels are usually screened as described herein to exclude panels with low withstand voltage. In detail, three layers, the first insulation layer 3, the luminescent layer 4, and the second insulation layer 5, are located between the first electrode 2 and the second electrode 6. Typically, EL display panels are designed so that a sufficient withstand voltage (life) exists as long as two layers remain, even if one of these three layers contains a defect (vacant area, or hole).
However, according to the study by the inventors of the present invention, it is difficult to guarantee a sufficient life for an EL display panel even though screening, as described above, is carried out, and it was found that short-circuits occurred between the first electrode 2 and the second electrode 6 within a relatively short time. The problem is described in detail with reference to FIG. 6A to FIG. 6C.
FIG. 6A to FIG. 6C, which show three cases, respectively, that may occur. FIG. 6A shows a case in which the second insulation layer 5 contains a defect. In this case, the second electrode 6 extends and is connected to the luminescent layer 4 through the defect of the second insulation layer 5, and two layers, namely the luminescent layer 4 and the insulation layer 3, remain between the electrodes 2 and 6.
FIG. 6B shows a case in which the first insulation layer 3 contains a defect. In this case, the luminescent layer 4 extends and is connected to the first electrode 2 through the defect of the first insulation layer 3, and two layers, namely the second insulation layer 5 and the luminescent layer 4, remain between the electrodes 2 and 6.
FIG. 6C shows a case in which the luminescent layer 4 contains a defect. Herein, the second insulation layer 5 extends and is connected to the first insulation layer 3 through the defect of the luminescent layer 4, and two layers, namely the second insulation layer 5 and the first insulation layer 3, remain between the electrodes 2 and 6.
These three cases may occur as described hereinabove, but according to the study by the inventors of the present invention, the lives of the displays of these three examples are different, and it was found that the life of the structure shown in FIG. 6A was significantly short in comparison with the other structures, experimentally.
In the case that the insulation layer 5 located between the luminescent layer 4 and the second electrode 6 in FIG. 6A contains a defect, it is important to avoid a short circuit between the first and second electrodes 2 and 6, for extending the life of the EL display panel.
The present invention has been accomplished in view of the problem found in the study conducted by the inventors of the present invention, and it is the object of the present invention to prevent short circuit between the first electrode and the second electrode through the defect of the insulation layer located between the luminescent layer and the second electrode of an EL element.
The invention is basically an EL element including a first electrode, a luminescent layer, and an insulation layer, and a second electrode. A defect hole is formed in the insulation layer. The layers are laminated successively on an insulative substrate. A cavity is formed in the luminescent layer under the defect hole, and the second electrode is separated from a layer located under the luminescent layer by the cavity.
The invention is further a method for forming an EL element in which at least a first electrode, a luminescent layer, an insulation layer, and a second electrode are laminated successively on an insulative substrate. The method includes laminating the layers on the insulative substrate and exposing the luminescent layer to etchant.